As recognized by tsuneto:98, external conditions are important
for reasoning about potential refinements of abstract plans. Although
the basic idea is the same, we define them a little differently and call
them external preconditions to differentiate them from other
conditions that we call external postconditions. Intuitively, an
external precondition of a group of partially ordered plans is a
precondition of one of the plans that is not achieved by another in
the group and must be met external to the group. External
postconditions, similarly, are those that are not undone by plans in
the group and are net effects of the group.
Definition 6 states that is an
external [pre, post]condition of an execution if is a
[pre, post]condition of a subplan for which it is not [achieved,
undone] by some other subplan.

For the example in Figure 2, is not an external precondition because, although G must exist to produce H, G is supplied by the execution of the
plan. Thus, is met internally,
making an internal condition.
is an external precondition, an
internal condition, and an external postcondition because it is needed
externally and internally; it is an effect of
which releases M1 when it is finished; and no other plan in the
decomposition undoes this effect.